Processes for upgrading low rank coal by using low quality bridging liquids, such as heavy oil, are well-known in the art. Canadian Patent No. 1,216,551 (Ignasiak) issued Jan. 13, 1987, is directed towards a method for agglomerating sub-bituminous coal using heavy oil. For such processes, quantities of heavy oil, in the order of 10 to 50 percent of the weight of the coal, are typically utilized. Coal is upgraded by selectively reducing the ash content and concentrating the coal.
Further processes have been developed to recover a portion of the heavy oil utilized in the agglomeration process. U.S. Pat. No. 4,854,940 (Janiak et. al.) issued Aug. 8, 1989, is directed towards a process for separating distillable hydrocarbons from sub-bituminous coal which has been agglomerated with a low quality bridging liquid by contacting the agglomerates with steam or nitrogen at temperatures between 250.degree. C. and 350.degree. C. Although a portion of the bridging liquid is recovered from the agglomerates by this process, the invention is directed primarily at the upgrading of the coal as a solid fuel.
Given that lighter, upgraded oils are typically more valuable than either upgraded coal or heavy oils, it is economically desirable to process mixtures of coal and heavy oil in such a manner that the heavy oil is recovered and also upgraded. Canadian Patent Application 2,022,721-4 (Ignasiak et. al.) filed Aug. 3, 1990, describes a method for recovering oil from coal fines that are agglomerated or mixed with heavy oil. The method involves heating the agglomerates or mixed coal fines to temperatures above 350.degree. C., up to 450.degree. C., in an inert gas atmosphere, such as steam or nitrogen, and condensing and collecting the oils distilled therefrom. The temperature range for the method is chosen to maximize the yield of distillable components while at the same time minimizing the generation of coal tars, which result from thermal cracking of the constituent hydrocarbons. Thermal cracking results in hydrocarbon products which require hydrotreating or hydrogenation before they are in a stable form, such that they are considered commercially acceptable, and are therefore not economical.
U.S. Pat. No. 4,396,396 (Mainwaring) issued Aug. 2, 1983, and U.S. Pat. No. 4,415,335 (Mainwaring et. al.) issued Nov. 15, 1983, utilize the well-known concepts and principles of vacuum and steam stripping to distill and separate out hydrocarbons from agglomerates or heavy oil/coal mixes. Stripping reduces the process temperature and increases oil yield while minimizing the effects of thermal cracking.
In the various processes used to separate hydrocarbons, the agglomerates and heavy oil/coal mixes typically contain large percentages of water which must be heated from ambient temperatures. In addition, the water contained in the agglomerates and mixes must be substantially removed in order for the agglomerates and mixes to be capable of reaching the temperatures required by these processes. Typically, as the agglomerates and mixes are heated from ambient temperatures, the water content is evaporated and the agglomerates and mixes are substantially dewatered. The dewatering of the agglomerates and mixes typically results in a significant loss of hydrocarbons with the vented steam. As well, when the agglomerates and heavy oil/coal mixes are heated to the temperature of the boiling point of water at the ambient pressure, the agglomerates and mixes can exhibit a first stage of stickiness, believed to be associated with the water, and adhere to the walls of the processing vessel, which may render them more difficult to process.
Further, in conducting these various processes to separate hydrocarbons, it has been found that the agglomerates and heavy oil/coal mixes exhibit a further or second stage of stickiness, believed to be associated with the heavy oil, following the evaporation of the water contained in them, during subsequent heating and stripping processes. Specifically, it has been found that the second stage of stickiness of the agglomerates and mixes is exhibited throughout a sticky temperature zone which is typically between about 180.degree. C. and 260.degree. C. Therefore, as the agglomerates or mixes are further heated, they become sticky and adhere to the walls of the processing vessel, which may render the agglomerates and mixes more difficult to process and result in plugging of the equipment used in the process.
As a result of the first and second stages of stickiness exhibited by the agglomerates and heavy oil/coal mixes during heating, the installation of mechanical scraping devices and other specialized equipment may be required. Such equipment may damage the processing equipment and may be difficult to maintain. In addition, the equipment utilized in these processes is often not thermally efficient and may therefore not be commercially viable.
As a result, there is a need in the industry for an improved process for upgrading a low rank coal and a heavy oil to produce both an improved coal product and an improved oil product while minimizing both the loss of hydrocarbons and the effects of the first and second stages of stickiness that may be exhibited while performing known upgrading processes.